1. Field of the Invention
The invention relates to the use of cationic surfactants in slurry-less environments during chemical mechanical polishing (CMP) of silicon dioxide wafer surfaces to substantially eliminate or reduce scratches and defects produced during the CMP process.
Certain categories of cationic surfactants chemisorb onto the silicon dioxide portion of the wafer during VLSI processing under all pH conditions, namely, acidic, neutral and basic pHs.
2. Description of the Prior Art
In recent years new chemical mechanical polishing (CMP) techniques have come into use that use a slurry-less polishing scheme for silicon wafer surfaces where the polishing pad has abrasive particles embedded therein. In particular, the polishing pad contains CeO.sub.2 as the abrasive material for silicon dioxide CMP applications.
During the CMP process, the wafer to be polished is abraded against the pad surface in an aqueous environment. However, during the CMP process at neutral or alkaline conditions these polishing pads produce defects and scratches, thereby causing yield losses.
Accordingly, it is desirable to devise compositions for and methods of, protecting the surface of silicon dioxide wafers from scratches and defects occasioned by abrasion during the slurry-less CMP process.
Somasundaran et al. "Surfactant adsorption at the solid-liquid interface Dependence of mechanism of chain length", J. Phys. Chem., Vol.68, pp.3562-3566 (1964), disclose that:
a) the concentration of surfactant required to neutralize the surface charge on silica decreases with increasing chain length; and PA1 b) from the critical concentration to neutralize the surface charge (as observed indirectly from a zero zeta-potential measurement for silica particles in an aqueous solution), the van der Waals energy scales is 0.97 kT per CH.sub.2 group or 580 cal/mole. PA1 Alkyl-trimethyl-ammonium halides; alkyl-benzyl-dimethyl-ammonium halides; pyridinium-alkyl halides; alkyl-ammonium esters; and mixtures thereof. The aniomic group could be varied to obtain the respective cationic surfactant ion.
On the basis of the Somasundaran et al., reference it would appear that there is a lateral hydrophobic bonding between the alkyl chains that are adsorbed on the silica surface. U.S. Pat. No. 5,769,689 discloses compositions for a method of polishing a workpiece comprised of silicon dioxide wherein the surface of the workpiece is exposed to a polishing composition comprising water, submicron SiO.sub.2 particles, a soluble inorganic salt at a concentration below the critical coagulation concentration for the composition, and wherein the pH of the composition is adjusted to within a range of about 9-10 by the addition of a soluble amine.
Kumar et al., "CHEMICAL-MECHANICAL POLISHING OF COPPER IN GLYCEROL BASED SLURRIES", Mat. Res. Soc. Symp. Proc. Vol. 427, pps. 237-242 (1996), discloses the use of a slurry containing glycerol and Al.sub.2 O.sub.3 abrasive for CMP processing of a copper metal surface to render the copper suitable as an interconnect fabrication. The process is an alternative to reactive ion etching of copper.
A chemical-mechanical polishing technique for LSI's to address the problem of dishing is disclosed in U.S. Pat. No. 5,607,718, in which a number of compounds are added to decrease the solution velocity and suppress dishing. These compounds include a number of thickeners such as silicates, glucose, tragacanth gum and agar.
Nojo et al., "SLURRY ENGINEERING FOR SELF-STOPPING, DISHING FREE SiO.sub.2 -CMP", IEDM, pages 349-352, .COPYRGT. 1996 IEEE, disclose a slurry in which a surfactant was added to a conventional CeO.sub.2 slurry for SiO.sub.2 in chemical mechanical polishing (CMP) to obtain global planarization within a chip without any stopping layer or design limitations.
U.S. Pat. No. 4,588,474 discloses a cleaning treatment on a metal surface wherein glycerin is added to the solution along with other additives to obtain a smoother and more uniform surface. These solutions contain a caustic, a nitrate or nitrite, and optionally, a diol or polyol such as ethylene glycol or glycerin, and the workpiece is an aluminum alloy.
An electrochemical polishing technique for metal removal and planarization of various components in various stages of microelectronic fabrication is disclosed in U.S. Pat. No. 5,567,300. This patent is directed to feature removal, and uses a non-viscous polyol to enhance removal. Glycerin, which has a viscosity at room temperature of 1500 times that of water, is used in electroetching to increase the electrolyte viscosity and promote polishing.
A significant disadvantage of the processes in the foregoing patent and literature references is that, there is no acknowledgment of the need to improve defect control of silicon dioxide chemical mechanical polishing (CMP) in a slurry-less system through chemistry modifications to the aqueous medium.